1. Field of the Invention
The invention applies to satellite communications and more particularly to a broadband access system using a constellation of satellites in low Earth orbit to provide broadband services, essentially multimedia services.
The invention applies to access systems using geostationary satellites (GEO) and also to access systems using satellites in low Earth orbit (LEO).
2. Description of the Prior Art
The invention applies very particularly to a new broadband access system that is currently under development. This system is called SkyBridge. It will make available to users worldwide services such as fast access to the Internet and videoconferencing. It will use a constellation of 80 LEO satellites which link business and domestic users equipped with low-cost terminals to terrestrial gateways.
The SkyBridge system has two segments:
A space segment: the space segment comprises 80 LEO satellites (plus spare satellites) in orbit at an altitude of 1457 km and the ground control segment comprising the satellite control center and tracking, telemetry and control stations. The space segment provides permanent coverage in the band of latitudes from +68.degree. to -68.degree.. It connects each SkyBridge user to the nearest gateway. PA1 A terrestrial segment: the terrestrial segment comprises the terrestrial gateways and the user terminals. The former provide interconnection via an ATM switch with local servers and with broadband and norrowband terrestrial networks. PA1 constructing an interference graph, PA1 use of the graph by a search algorithm associated with a constraints propagation algorithm to verify the capacities.
The SkyBridge access system is based on the asynchronous transfer mode (ATM) used to connect users to a local switch. Traffic emanating from terminals is transmitted in a transparent manner by the satellite (i.e. without any processing other than amplification and frequency conversion) to the gateway and vice versa. The gateways have switching functions and serve as interfaces with terrestrial networks.
The Earth is divided geographically into areas with a radius of about 350 km which comprise up to 2,000,000 potential clients and a gateway between those clients and the satellites of the constellation which illuminate the area. The station enables clients of the service in that area to communicate with the constellation.
The satellites of a low Earth orbit constellation generally move across the sky and can illuminate a geographical area z defined in the above manner for at least a few minutes and at most around twenty minutes.
The access system must nevertheless provide a certain bit rate between the constellation and the fixed points that the gateways form in this environment.
To this end, it is necessary to optimize the resources available on board each satellite. A way must be found to allocate the resources to points on the ground so that the services offered can be provided and meet the demand from clients.
The resources for a satellite are the number of antennas or individual beams multiplied by the number of channels (frequency resource) available to each satellite in the frequency band reserved to the access system. The generic term "links" will be used in this context.
One way of increasing the number of frequency bands that can be used in the band reserved for the system is to transmit with one or other polarization (which in practice doubles the number of frequency bands in the band reserved to the system). However, this still means that the same channel must not be used for neighboring areas.
A first constraint is then encountered and is due to the problem of interference between waves when using the same frequency band for adjoining areas. Using the same frequency band for neighboring areas (i.e. adjacent or closely spaced areas) causes interference which reduces the signal to noise ratio of the received wave. In this case it is therefore essential not to use the same frequency band for said areas.
Until now this problem has been solved by using link allocation plans fixed over a given period, established for several days and used by all the satellites of the constellation. These plans are communicated to them periodically by a processing management station on the ground.
A solution of this kind is entirely acceptable for existing access systems, i.e. systems with fixed antennas.
For future systems with steerable antennas (steered electrically or mechanically), this solution is not satisfactory because it does not exploit all the resources offered by the dynamic aspect of such systems.
This is the case in particular with the new SkyBridge system with mechanically steered antennas.
The satellites of the constellation of this system will use steerable antennas to produce individual beams illuminating the areas of the system and to keep them pointing toward the corresponding gateways. Each satellite will illuminate a region with a radius of 3000 km.
It is therefore necessary to optimize the allocation of links at all times to take account of additional possibilities offered by steerable antennas, and the prior art solution is therefore no longer acceptable.
The invention solves this problem.